Active object detection systems use active signaling to detect objects, such as objects within a defined scanning range or monitoring area. Active signaling examples include the emission of laser light or other electromagnetic energy. In general, if an object is sensed to be within a predefined area, then some action is taken by a control circuit within the detection system. The particular action taken by the system may be a function of the detected distance to the object. Active object detection systems therefore commonly include distance-determining mechanisms, such as “pulsed time of flight” (TOF) measurement circuits.
With TOF-based distance determination, the system emits a pulse of light along a defined beam path, and the corresponding return reflection is detected by a photo-receiver within the system. Elapsed timing determination, where the amount of time between the outgoing light pulse transmission and the return reflection pulse reception is determined with high precision, provides the basis for accurate distance measurement. Often, the time delay is measured using specialized electronics, and converted into a distance, d, using the relation
      d    =          c      ⁢                          ⁢              t        2              ,where c=the speed of light and t=the round-trip time delay.
For scanning-based detection systems, the optical field of view often is rotated synchronously with the pulsed emissions, allowing angular measurements to be correlated to the distance measurements made through TOF. In other words, a scanning system of this type tracks the beam angle and flight time for each emitted pulse, which allows the system to determine the distance and angle of an object relative to the system.
U.S. Pat. No. 6,753,776 to Drinkard discloses a TOF-based laser scanner that includes a housing containing a rotating mirror assembly that sweeps a pulsed laser beam through a desired scanning angle. Return pulses are reflected by the same rotating mirror assembly into a receiver circuit. The TOF of each pulse is measured using a tapped delay line circuit described in U.S. Pat. No. 6,493,653 to Drinkard et al.
With the “tapped delay line” taught by the '653 patent, a laser pulse emission produces a start pulse that is input to a series chain of digital buffers. Each buffer propagates the start pulse to the next buffer in the chain, and each one triggers a corresponding digital capture register that samples a return reflection signal line. Thus, each capture register corresponds to a known depth or position within the buffer chain and, hence, to a known time offset relative to the beginning of the chain. Determining laser pulse flight times thus depends on inspecting capture register contents to determine which capture registers recorded the return reflection pulse. U.S. Pat. No. 6,665,621 to Drinkard et al. teaches advantageous approaches to waveform data processing, as relates to tapped delay lines of the type detailed in the '653 patent.
Regardless of the time-base circuit details, a general operational proposition of many such TOF-based scanners is that a laser pulse is emitted on a given beam projection, and a corresponding reflection is returned by the first object encountered along that beam projection. That proposition generally holds where the first encountered object is larger than the beam cross-section, such that it completely shadows any more distant objects lying behind it along the beam projection. Conversely, the proposition does not hold where the first encountered object is small enough to allow at least a portion of the laser pulse to pass by it. In such cases, multiple return reflections may be generated by a single output pulse; a first return reflection caused by the small object blocking a portion of the pulse, and one or more subsequent return reflections caused by more distant objects along the beam path. It will be understood that the more distant object(s) along the same beam path are illuminated by that portion of the beam passing by the nearer object(s).
Small, near-object reflections are problematic particularly where an active object detection system is required to detect faintly reflecting objects at long sensing ranges with guaranteed measurement accuracy. Such a scanner is very sensitive to small, nearer objects lying between the scanner and a more distant object to be detected. For instance, small airborne particles, puffs of smoke or transitory clouds of suspended dust (for instance concrete dust) may partially block the scanner's view of a more distant object. Because such detection systems are typically configured to respond to a “first object detected” criterion, such “clutter” generates false object detections, leading to unnecessary or inappropriate actions of the control circuit.
Known approaches to mitigating a scanner's susceptibility to clutter-related false detections include requiring the scanner to detect and track objects for two or more consecutive detection times, which may be “scan” related. Another approach requires the scanner to detect an object on at least two adjacent beam angles or directions. The former technique may be understood as a temporal or persistence-based qualification, while the latter technique may be understood as a spatial or size-based qualification. Either of these techniques, or a combination of them, can reduce false object detections associated with small particles flying through a scanning field; however, their effectiveness diminishes greatly for suspended dust particles, which may persist in the air, dispersing slowly compared to the scan time.
In another approach, the scanner operates with a higher reflected pulse detection threshold as a basis for filtering out unwanted clutter. Raising the detection threshold means that the typically weaker reflections characteristic of clutter are not detected by the scanner as object reflections. In other words, the scanner electronics do not “see” weaker reflections that are below an elevated detection threshold. Equivalently, one may also lower the system gain, which drives weaker pulses below a fixed threshold. While such techniques offer good clutter rejection performance, they come at the cost of decreased scanner sensitivity. Sensitivity reduction may not be tolerable, particularly in scanners that are required to reliably detect the faint reflections associated with distant and/or low-reflectance objects.
A modified approach to sensitivity-based clutter rejection adjusts or sets scanner sensitivity based on intended object detection ranges. Higher thresholds are used for shorter detection ranges, while lower thresholds that preserve the scanner's sensitivity are used for long detection ranges. Of course, the lowered detection thresholds used for longer-distance ranges leaves the scanner vulnerable to clutter-related false detection problems.